Spray drying apparatus



June 9, 1959 K SAUTER SPRAY DRYI'NG APPARATUS Filed Dec. 7, 1955INVENTOR Snr/ree ATTORNEYS SPRAY DRYING APPARATUS Karl Sauter, Uitikonam Albis, Switzerland, assigner to Lawa SA., Zurich, Switzerland, acorporation' of Switzerland Application December 7, 1955, Serial No.551,623

Claims priority, application Switzerland December 11, 1954 11 Claims.(Cl. 159-4) This invention relatesto spray drying methods and apparatusand more particularly relates to a method and apparatus for avoidingunwanted deposits which necessitate frequent equipment shut down forcleaning.

In the spray drying of liquids utilizing conventional centrifugal spraydrying apparatus wherein a rotating spray disc extends into` a drying ordessicating chamber, undesirable deposits of dry material graduallybuild up between the stationary and moving parts of the apparatus sothat eventually the moving parts rub against the iixed parts to slow thespeed, load the motor, and necessitate a shut down forv cleaning. Thesedeposits also tend to clog the orifices in the disc. In addition tocausing frequent shut downs for cleaning, these deposits tend to becomeburned by the high temperature of the spraying apparatus and, where suchdeposits fall into the dry product, contamination occurs and a largeamount of the product may be rendered unsuitable for human consumption.

While numerous yattempts have been made to solve these problems, such asthe use of a current of cold air, high pressure nozzles, or twosubstance nozzles, no completely effective remedy has been found and allequipment of this type must be periodically shut down for cleaning.

It is accordingly a primary object of the present invention to provide amethod and apparatus for spray drying wherein the accumulation ofundesirable deposits of the substance to be extracted is prevented andsubstantially continuous operation of the equipment is possible.

lt is another object of the invention to provide a novel method andcentrifugal atomizer apparatus wherein unwanted deposits are preventedfrom forming between the fixed and vmoving parts of the apparatus.

It is another object of the invention to provide an irnproved method andapparatus for spray drying liquids on a continuous basis withoutnecessity of shut down for cleaning deposits from the apparatus.

It is another object of the invention to provide a method and apparatusfor spray drying a substance carried by a liquid by spraying the liquidinto a drying atmosphere which tends to cause deposits of the substanceto form on certain surfaces of the spraying apparatus, while preventingthe formation of such deposits by projecting onto the deposit pronesurfaces a wet gas.

Further objects and advantages of this invention will become apparentupon reference to the following'description and claims and appendeddrawings wherein:

Figure 1 is a vertical section of a spray drying apparatus constructedaccording to one embodiment of the invention;

Figure 2 is a vertical cross section through a spray drying apparatusconstructed according to another embodiment of the invention; and

Figure 3 is a vertical section through a spray drying apparatusconstructed according to still another embodiment 'of the invention.

' ted States arent 2,889,873 Patented June 9, 1959V izing head or disc12 attached to the lower end thereofV by a nut 14. The shaft 10passesconcentricallythrough the housing 16 and is journalled therein bymeans of aA bearing 18 carried by supports 20 and 22 which are fixed tothe housing 16 by means of the wall 24. The

housing 16 is inserted into the dryerl container, shown partially at 26,which forms the atomizing chamber. Container 26 includes an aperture 27for delivering a drying gas to the container.

The oil for lubrication of bearing 18 may be delivered to the bearingfrom above and is thence conveyed past the splash ring 28 to the oilcatching bowl 30 located in bearing support 22. From here the oil isremoved by means of a suitable conduit, not shown. A labyrinth seal 32is mounted on the bearing support 22 to prevent the oil from runningdown along the shaft 10 into the drying chamber.

The liquid to be evaporated is carried through a pipe, not shown, toring duct 34 in bearing support 22 and from'here is delivered throughliquid supply tubes 36 into the cavity 38 in the atomizer disc 12. 'Ihering duct Se is otherwise closed by means of a cover plate 40 which isreceived on a shoulder 42 in the housing 16 and the housing extends downbelow the cover plateV to form a peripheral skirt 44 which terminates inclose proximity to the disc 12. The liquid supply tubes 36- are carriedby the cover plate 40. The atomizer disc 12 is dish shaped and isprovided with an upwardly extending rim 46 having radial nozzles 48therein. The upper portion of the rim 46 carries an inwardly extendingtlange 50 which serves as a means for mounting a covering ring S2.

The liquid entering the annular cavity 38 in the atomizer disc isprojected towards the rim under the influence of centrifugal force andis sprayed through radial nozzles 48 into the drying chamber, where itis emitted as a haze in the surrounding drying gas. As the liquid leavesthe supply tubes 36 and impinges upon the rapidly revolving disc, it ispartially dispersed and a considerable number of drops thereof enter thespace 54 between the covering ring 52 on the atomizing disc and thecover plate 40 in the housing 16. These drops adhere to the cover plate40 and covering ring 52 as well as the supply tubes 36 and tend to dryin the drying gas atmosphere which permeates this space. This actionresults in the formation of a constantly growing deposit whicheventually leads to a rubbing contact between the deposit on thecovering ring 52 and the deposit on the cover plate 40. This slows therotation of Ythe atomizing disc, overloads the driving motor, andeventually necessitates a shut down of the apparatus for cleaning.

In addition to the foregoing deposits, the atomizing disc tends tobecome heated to the temperature of the hot drying gas normally utilizedand the uid itselffrequently enters the apparatus at a high temperature.Under these conditions, and even under conditions where the liuid entersatroom temperature, the fluid leaving the supply tubes 36 and spreadingacross the bottom of the cavity 38 is immediately heated, so that acertain amount of evaporation occurs inside the disc. This leads to thelaccumulation of deposits in the vicinity of the supply tubes 36 and theshaft 10 and these deposits grow until they contact the tubes 36 tocause eventual slowing of the disc, loading of the motor and shut downfor cleaning. Further, the high temperature of the disc causes a burningof the deposits so that if any of the` deposits, shut downs andcontamination are avoided by means of a wet gas cushion which preferablycontains a vapor of the same liquid as carries the substance to beextracted. This vapor is preferably present in an amount suicient tocreate a partial pressure of the vapor in the gaseous mixture equal toor higher than the saturated vapor pressure of the particular liquidwhich carries the substance to be extracted. By this means carrierliquid evaporation on the surface of the spraying apparatus iseliminated and there is consequently no formation of deposits.

As an example, where milk is to be spray dried, water is the materialbeing evaporated and hot air at a temperature of approximately 140 C. isgenerally used as the drying gas. Under the influence of this air andthe inilowingl liquid the atomizer disc attains a tem-V perature ofapproximately 50 C. which is immediately communicated to the milk on thedisc. In order to prevent evaporation the gas cushion should consist ofair at a temperature of approximately 50 C. containing a quantity ofwater vapor or steam at a temperature of at least 50 C. and in an amount-suthcient to create a partial pressure equal to the saturated vaporpressure of water at 50 C.

In the apparatus shown in Figure 1 the proper mixture of cushion gas isobtained in an annular chamber 56 into which air is admitted by means ofconduit 58 and steam is admitted by means of conduits 60 and 62 havingoutlets 64. The steam or water vapor may be produced in any suitablemanner, such as in a boiler, and then introduced into conduits 60 and62. Where the desired temperature of the water vapor is relatively low,as in the foregoing example, it is obvious that some reduction in thetemperature of the steam is necessary, with the partial pressure of thevapor being controlled by the amount of water vapor introduced into thecushion gas. Apertures 66 are provided in the cover plate 40 and theseadmit the cushion gas into the space 54 between the cover plate 40 andthe covering ring 52. Since the cushion gas is preferably supplied at apressure slightly in excess of the pressure of the drying air in thedrying chamber it expels the drying air from the space 54 andeffectively eliminates evaporation of liquid and build up of deposits inthis area. The small clearance between the peripheral skirt 44 and disc12 facilitates this action and prevents any inflow of drying gas.

The continuous ilow of cushion gas and the centrifugal eiect of therapidly rotating disc 12 causes the gas to flow radially outward overthe rim 46 of the disc. In addition to maintaining a saturatedatmosphere in the space 54 this flow tends to reduce the quantity of hotdrying gas present at the nozzles 48 and produces an annular zone aroundthe nozzles having at least a greatly reduced drying potential. Thisreduces the formation of deposits at the nozzles and permits longoperation without shut down for cleaning. This effect may be furtherenhanced by utilizing a housing 16 of larger diameter than that shown inFigure l whereby the peripheral `skirt 44 extends downwardly to surroundthe upper portion of the rim 46. With such a construction the radiallyescaping cushion gas is brought into the immediate vicinity of thenozzles to almost completely eliminate the presence of drying gas atthis point.

In order to limit the centrifugal dispersing action of the rapidlyspinning disc 12 on the cushion gas, the undersideof cover plate 40 isprovided with radial vanes 68, two of which are shown in cross sectionin Figure l, which tend to limit the rotational movement of the gas. Inaddition, the clearance between the lower edge 44 of the housing 16 andthe upper surface of the rim 46 of the disc 12 is preferably kept at aIt will be appreciated that in the spray driers heretofore utilizedsmall clearances of this type have been impractical, since the rapidaccumulation of deposits at these points caused shut down of theapparatus after only a short period of operation.

While the apparatus shown in Figure 1 illustrates one type of spraydrier incorporating the invention, the principle of the invention may beapplied to a wide variety of spray driers and a second embodiment ofapparatus is shown in Figure 2. In the apparatus shown in Figure 2 theshaft 70 passes through a labyrinth seal 72 carried by the cover plate74 which closes the lower end of the housing 76. A dish shaped spraydisc 7S having an upstanding peripheral rim is attached to the shaft 70by means of a nut 82. -The upper opening in the spray disc is closed bymeans of a covering ring 84 which engages the rim 80 and the rim isprovided with spray nozzles 86. The iiuid to be spray dried ows into thecavity 88 in the spray disc through fluid supply tubes 90 which passthrough the cover plate 74. A centrally located circular skirt 92depends from the underside of cover plate 74 into close proximity to thecentral opening in the covering ring 84. The covering ring is providedwith a shoulder 94 at this position and the shoulder defines passageways96 for a purpose presently to become apparent.

Cushion gas of the proper temperature and liquid content is conducted bypipe 98 into an annular duct 100 surrounding the shaft 70. From here thecushion gas escapes into the space 102 enclosed by the skirt 92 past thesupply tubes 90 into the cavity 88 in the spray disc 78. The labyrinthseal 72 precludes the cushion gas from rising along the shaft 70 intothe lubricating elements.

An additional supply of cushion gas is provided by means of pipe 106which supplies gas through cover plate 74 into the space 108 between thecover plate 74 and covering ring: 84. The supply of cushion gas downthrough the space 102 within skirt 92 and into the spray disc preventsthe formation of deposits in the vicinity of the supply tubes 90 and inthe cavity 88 in general. The cushion gas from pipe 106 prevents theformation of deposits within the space 108 while the small clearancebetween skirt 92 and collar 94 prevents liquid from escaping into thespace 108. Any droplets which do tend to escape are delivered back intothe cavity 884 within the spray disc 78 by centrifugal action throughthe passages 96. It will be appreciated that the small clearance betweenthe skirt 92 and collar 94 is possible only because a positive pressureof cushion gas exists within the spray disc and prevents the formationof deposits which would soon iill this small space.

Cushioning gas may be supplied to the vicinity of the nozzles 86 bymeans of a passageway 110 in shaft 70 which communicates with an annularduct 112 having radial passages 114 connected thereto and terminating atthe nozzles 86.

In addition to being applicable to rotating d1sc type sprayers, theprinciple of the invention may also be applied to fixed spray nozzlesand an illustration of such an arrangement is shown in Figure 3. Liquidto be evaporated is introduced into the drying vessel 116 by means of aconduit 118 which is connected to nozzle assembly 120 having a nozzleorifice member 122. The nozzle assembly 120 is surrounded by a jacket124 which is sealed to the nozzle at 126 by any suitable packing means.The lower end of the jacket 124 adjacent the nozzle oriiice member 122is open, as shown at 128, and cushion gas is supplied to the jacketthrough a conduit 130 in the upper end thereof. The nozzle orificemember 122 and the opening 128 in the jacket are so arranged that thejacket does not interfere with the dispersion process but prevents theliquid which gathers around the nozzle orifice from drying out.

While the cushion gas has been described as consisting of a mixture of agas, such as air, and a liquid which is the same as the liquid carryingthe substance to be extracted, it is also possible to increase thepercentage of the liquid phase of this mixture to the extent that itcomprises the entire cushion gas. That is to say, during a spray dryingprocess wheren water is the liquid to be evaporated, the cushion gas mayconsist of pure steam. In such va situation a very small quantity ofcushion gas is effective to achieve the desired purpose, although it isnecessary to take care lest the temperature of the spray disc be raisedan undesirable amount.

It will be apparent from the foregoing that there has been provided amethod and apparatus for spray drying liquid borne substances wherebythe accumulation of dry deposits of such substance on the moving andstationary parts of the apparatus is eliminated so as to permit a trulycontinuous operation. This effect is achieved by blanketing the areaslikely to accumulate deposits with a cushion gas which includes anamount of the liquid bearing the substance to be extracted suiiicient togive a partial pressure equal to the saturation pressure of that liquid.

The invention may be embodied in other specific forms Without departingfrom the spirit or essential characteristics thereof. The presentembodiment is therefore to be considered in all respects as illustrativeand not restrictive, the scope of the invention being indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are therefore intended to be embraced therein.

What is claimed and desired to be secured by United States LettersPatent is:

1. A method of spray drying a substance carried by a liquid comprisingthe steps of spraying said liquid into a drying atmosphere which tendsto cause deposits of said substance to form on certain surfaces of saidspraying apparatus, and preventing the formation of such deposits byprojecting onto the deposit prone surfaces a gas containing vaporchemically similar to the vapor of said liquid in suicient quantity toproduce at said surfaces a mixture having a partial vapor pressure ofsaid chemically similar vapor substantially no less than the saturatedvapor pressure of said liquid at lthe temperature of said mixture.

2. A method of spray drying a substance carried by a liquid comprisingthe steps of spraying said liquid into a drying atmosphere which tendsto cause deposits of said substance to form on certain surfaces of thespraying apparatus, and preventing the formation of such deposits byprojecting onto the deposit prone surfaces a gas containing a vapor ofsaid liquid, the partial pressure of the vapor of said liquid being atleast equal to the saturation vapor pressure of the liquid carrying saidsubstance.

3. A method of spray drying a substance carried by a liquid comprisingthe steps of spraying said liquid into a drying atmosphere which tendsto cause deposits of said substance to form on certain surfaces of thespraying apparatus, and preventing the formation of such deposits byprojecting onto the deposit prone sirfaces a gas containing a vapor ofthe same liquid as the liquid carrying said substance, the partialpressure of said vapor being at least equal to the saturation vaporpressure of the liquid carrying said substance.

4. A method as set out in claim 3 wherein said gas is air and said vaporcarried by said gas is water vapor.

5. A method of spray drying a substance carried by a liquid comprisingthe steps of spraying said liquid in a spray drying apparatus in adrying atmosphere which tends to cause deposits of said substance toform on certain surfaces of the spray drying apparatus, and preventingthe formation of such deposits by projecting onto the deposit pronesurfaces a gas containing a vapor of said liquid to form at saidsurfaces a mixture, the partial pressure of said vapor being at leastsubstantially equal to the saturated vapor pressure of the liquidcarrying said substance at the temperature of said mixture.

6. A spray drying apparatus for drying a substance carried by a liquidcomprising a spraying means, a container enclosing said spraying means,means for delivering to said container a drying gas Which tends to causethe formation of deposits of the substances to be dried on surfaces ofsaid spraying means, means for providing a gas containing vaporchemically similar to the vapor of said liquid and means for deliveringsaid gas containing vapor to said surfaces, said means for providing agas containing vapor providing said gas and vapor in such proportionsthat a mixture is produced at said surfaces having a partial vaporpressure of said chemically similar vapor substantially no less than thesaturated vapor pressure of said liquid at the temperature of saidmixture.

7. An apparatus as set out in claim 6 wherein said spraying means hasfixed and rotatable portions adjacent one another, said rotatableportion comprising a dishshaped disc having a turned up rim with nozzlestherein, a ring plate covering said disc, a cover plate on said fixedportion parallel to said ring plate, means for delivering said liquidpast said cover plate and ring plate into said disc near the axisthereof, and conduit means in said cover plate for delivering said gascontaining vapor into the space between said cover plate and said ringplate.

8. An apparatus as set out in claim 7 wherein said fixed portionincludes therein a mixing chamber for mixing said gas and Vapor, andfurther conduit means connecting said mixing chamber to said conduitmeans.

9. An apparatus as set out in claim 6 wherein said spraying meansincludes fixed and rotatable portions adjacent one another, saidrotatable portion comprising a dish-shaped disc having a turned up rimwith nozzles therein, a ring plate covering said disc, a cover plate onsaid fixed portion parallel to said ring plate, means for deliveringsaid liquid extending through said cover plate past said ring plate intosaid disc near the axis thereof and conduit means in said cover platefor delivering said gas containing vapor into the space between saidcover plate and said ring plate, said fixed portion including a lowerperipheral edge which extends down towards and into close proximity withsaid disc below said cover plate to thereby form a small slit whichmaintains said gas in said space and prevents surrounding gas fromentering.

10. An apparatus as set out in claim 9 including radial vanes affixed tothe underside of said cover plate.

11. A spray drying apparatus for drying a substance carried by a liquidcomprising a spraying apparatus having fixed and rotatable portionsadjacent one another, said rotatable portion having a cavity thereincommunicating with nozzles, first conduit means in said xed portion fordelivering said liquid to said cavity in said rotatable portion, amixing chamber in said fixed portion, second conduit means in said fixedportion connected to said mixing chamber for delivering a gas thereto,third conduit means in said fixed portion for delivering a liquid tosaid mixing chamber, and fourth conduit means in said xed portion fordelivering a mixture of said gas and'second named liquid to surfaces ofsaid rotatable portlon.

References Cited in the file of this patent UNITED STATES PATENTS1,634,640 Zizinia .Tuly 5, 1927 2,367,770 Hall Ian. 23, 1945 2,584,973Andermatt Feb. 12, 1952 FOREIGN PATENTS 64,167 Denmark Dec. 3, 1945OTHER REFERENCES Heat Power Engineering, part I, 3rd edition, Barnard,Ellenwood and Hirschfeld, pp. 84, 269, published by John Wiley & Sons,Inc.

